As congestion on the Northern Line increased in the '30s, a plan was developed to build a second pair of tunnels in parallel with the Charing Cross branch of the Northern Line that would act as an express route through London. These plans were shelved at the outset of the Second World War, but as the platforms of the Underground became increasingly used by the general public overnight as air raid shelters (despite being initially discouraged), work began in 1940 on building deep level shelters which were envisaged to eventually become the platform tunnels for the express route.
Ten shelters were originally planned - five to the north of the Thames and five to the south. All of these were to eventually form part of the Northern Line express route apart from Chancery Lane and St. Pauls, which were associated with the Central Line. The stations on the Northern Line that were equipped with extra tunneling are: Belsize Park, Camden Town, Goodge Steet, Stockwell, Clapham North, Clapham Common, Clapham South. Work on St. Pauls was abandoned in 1941 for fear of damaging the cathedral's foundations and also work ceased on a shelter at Oval soon after, due to extensive flooding.
Each shelter was originally designed to house up to 12,000 people but by the time they were built, the number of bunks had been dropped to a more comfortable 8,000. Bunks were arranged along the walls in various configurations, to maximise use of space.
Most of the shelters took about a year and a half to complete, but amazingly, the government got cold feet about using them as public shelters as they were incredibly expensive to maintain. Several of the shelters were re-purposed. Goodge Street became Genereal Eisenhower's headquarters, some were converted for government use while others were used as army barracks, to house troops in transit.
It took the advent of the flying bombs (V1 and later V2) to convince the government that they should be used as they were originally intended and some were converted back to be used as public shelters.
After the Second World War came to an end, plans to create the express route stayed on the shelf, and were then ultimately dropped as money for the project wasn't available. Most of the shelters found post war use initially as accommodation for the army in transit and most are today in use as storage facilities.
